With the development of power system, the new techniques based on power electronics were more widely used in power system like DC transmission technology, flexible AC transmission technology, and digital substation technology and so on. In power system, the need is more urgent than ever before for practical optical current/voltage sensor (hereinafter referred to as fiber optical current sensor, optic PT). At present, the high potential end usually adopts electronic circuit active fiber optical current sensor, optic PT. The practical operation indicated that the fiber optical current sensor, optic PT exist safety problem, and should research the passive fiber optical sensor, optic PT for the high optional end.
Since the 60's of 20th century, lots of passive fiber optical current sensor concepts and designs were proposed. These concepts and deigns make use of the optical effects produced by current like magnetic energy, electric energy, heat energy, mechanical energy and so on, adjust amplitude of light (intensity), frequency, phase, polarization state. The effects applied included magnetic circular birefringence or magnetic rotation effect (polarization state) known as Faraday Effect, magneto-birefringent effect (polarization state) known as Cotton-Mouton Effect, magnetostrictive elasto-optical effect (phase), current thermal effect—heat up cold shrink effect—elasto-optica effect (phase), magnetostrictive—Bragg Diffraction Effect (frequency), current thermal effect and Rayleigh scattering, Raman scattering (intensity). Later, the focus was the Faraday Effect designs and concepts, and particularly the Faraday Effect fiber optical current sensor was gradually concentrated to the polarimeter and interferometer sensor two designs and concepts.
The method of the polarimeter fiber optical current sensor measuring determined the measured current is by measuring the intensity of two output lights the polarization plane perpendicular to each other, and determining the rotation angle of optical polarization plane produced by measured current, and then determining the measured current. This fiber optical current sensor related to linearly polarized light only. The optical loss and additional noise is tiny. The bandwidth of pre-amplifier need is identical to measured signal, e.g. 5 kHz. The signal processing circuit is simple. However, the signal processing circuit of this fiber optical current sensor accepts optical loss passively only, and can't interference its work (1980, several polarimeter closed-loop fiber optical current sensor were proposed, but they all were abandoned owing to slow response speed and large power consumption in principle, see literature [1, see IDS]). In 1966, University of Tokyo of Japan developed the “Laser Current Transformer” [2]. In addition, in 1978 china showed the “laser current sensor” developed by China Electric Power Research Institute at the National Science Conference Exhibition. And English and Chinese scholars joint developed fiber optical double beams, four output lights complex system [3], and ABB firm developed bulk optical media-fiber optical transmission double beams, four outputs polarimeter fiber optical current sensor[4]. None of them can solve precision and long-term stability problem due to environment temperature, vibration induced birefringence.
Considering above case, the relevant scholars proposed a phase modulation Sagnac interferometer fiber optical current sensor [6], [7], which is based on existing Sagnac interferometer fiber optical current sensor, and referred to digital closed-loop fiber optic gyro military technology. As FIG. 1 showed, in this design scheme, except for sensor fiber circle, the transmission lights are same amplitude, and the polarization plane perpendicular to each other two linear polarization lights; each light passes a 45° λ/4 wave plate and convert two same amplitude and reversed direction rotation circular polarization lights, then enter sensor fiber circle. The Larmor precession, which is produced by measured current in the sensor fiber circle, cause a different frequency shift of circular polarization lights that enter sensor fiber circle and its rotation direction is identical and reversed to electron precession direction, and leads to the phase difference of two circular polarization lights (The Faraday Effect derived from this). The two circular polarization lights, which come from sensor fiber circle pass through the 45° λ/4 wave plate again and return the transmission light path, restore to polarization plane perpendicular to each other two linear polarization lights, and the phase difference of circular lights becomes that of linear lights. Those produce interference with the polarizer that is 45° angle with the two linear polarization lights. Therefore, the measurement of the current magnate field is become the measurement of the phase of linear light, and this can achieve by the mature interference technology because the precise measurement of the phase of light wave is much easier than that of polarization state [8]. In theory, this fiber optical current sensor is more precise than common polarimeter fiber optical current sensor. Because linear polarization phase is easy to feedback voltage light wave phase modulator, so this fiber optical current sensor is easy to realize digital closed-loop, and benefit to improve system stability and the measurement precision.
The digital closed-loop Sagnac interferential fiber optical current sensor attracted many attentions as proposed, and lots of companied and corporations began to develop it [9-19]. In china, China Electric Power Research Institute, Beihang University, Department of Space and other units have also joined in this research. But there are some problems of this kind of fiber optical current sensor, that polarization model of optical fiber current sensor does not.
First, this kind fiber optical current sensor used λ/4 wave plate to realize the wave polarization state conversion, linear polarization—circular polarization—linear polarization. This leads some problems. At present, the λ/4 wave plate usually was made of a kind of high birefringent fiber with 90° phase difference of its fast and slow axis. But this fiber phase delay temperature coefficient is larger, about 2×10-4/° C. usually. Therefore, the delay phase of this λ/4 wave plate will deviate 90° due to the temperature change. It cannot realize the wave polarization conversion of linear polarization-circular polarization-linear polarization. This leads the modulation efficiency of the light signal [8],[10] (see U.S. Pat. No. 5,987,195), and change the ratio of the fiber optical current sensor, and increase the noise signal (see U.S. Pat Nos. 7,038,718, 5,987,195).
Second, the light path phase modulation and the signal processing of the circuit are both dependent on square wave closed-loop strategy. This reduced the measurement precision in the tiny signal range, also narrowed its dynamic range, slowed down the response speed of the fiber optical current sensor. Different from the polarization fiber optical current sensor, the closed-loop fiber optical current sensor processed the square wave carrying signal information instead of the optical signal with the similar waveform; and mixed strong peak spikes caused by square wave modulation (see U.S. Pat. Nos. 5,684,591, 5,280,339). Because the phase difference of two circular polarized lights induced by measured current magnetic field was small, the square wave amplitude of the signal current was nanoampere level only when the total output optical current was microampere level. But the bandwidth of the amplitude modulation square wave was very wide. E.g. when the bandwidth of the signal was 5 kHz, the bandwidth of the amplitude modulation square wave was 6 MHz above at least. It is the 1200 times than that of the polarimeter fiber optical current sensor under the same condition. Only this point, under the same output noise current density condition, the noise power level of output signal of the preamplifier of the digital closed-loop fiber optical current sensor was 1200 times than that of the polarimeter fiber optical current sensor. For this reason, this kind of fiber optical current sensor not only detected the square wave amplitude, produced the feedback stepped slope wave, modified feedback gain and other digital closed-loop calculations, and also need to carry out a large amount of noise reduction calculations. Moreover, noise point data results were associated with large amounts of data before and after this point the (in theory, should be associated with global data). Therefore, the delay time (not the calculation time associated with the device operation speed) of the method was long, and its response speed was slow. And the latest U.S. patent “Optical sensor, optical current sensor and optical voltage sensor” (application number: 20090212763) considered “it is difficult to improve the response speed” as one of the problems of this kind fiber optical current sensor.
For the λ/4 wave plate phase delay change with the temperature, we can use our designed zero temperature coefficient optical fiber wave plate to solve this problem. For details, see our pending patent “zero temperature coefficient of the optical wave plate and polarization state converter”. The present patent is to solve the second problem above mentioned.